1. Field of the Invention
The present invention relates to a driving method. More particularly, the present invention relates to a driving apparatus of a light-emitting diode and a driving method thereof.
2. Description of Related Art
Light emitting diodes (LEDs) have advantages of small size, power-saving and high durability, and as fabrication processes thereof become mature, price of the LEDs decreases. Therefore, it is popular to use the LEDs as light source products. Moreover, since the LED has features of low-operating voltage (only 1.5-3V), initiative light-emitting, and having a certain brightness, wherein the brightness can be adjusted by voltage or current, and has features of impact resistance, anti-vibration and long lifespan (100,000 hours), the LED is widely used to various terminal equipments, such as vehicle headlamps, traffic lights, text displays, billboards and large screen video displays, and domains such as general level architectural lighting and liquid crystal display (LCD) backlight, etc.
Regarding a driving circuit of the LED, a commonly used dimming method thereof is to regulate a duty cycle of a pulse according to a pulse-width modulation (PWM) technique, so as to regulate an equivalent current output to the LED by an output stage to adjust a brightness of the LED. However, when the PWM technique is used for dimming, a current switching operation of the output stage is the same as that of a switch. The current switching operation lead to a great load variation of a voltage of the output stage, so that the voltage may have an excessive ripple. Meanwhile, the excessive ripple can cause a great magnetic field variation of an inductor in the circuit, and a capacitor in the circuit can be sharply vibrated to generate a shape-changing due to an excessive transient voltage variation, so that an audio noise is generated.
FIG. 1A is a system schematic diagram illustrating a conventional driving circuit of an LED. Referring to FIG. 1A, the driving circuit 100 includes a voltage converter 110, a conversion loop controller 120, an amplifier 130, a voltage selector 140 and a current driving unit 150 formed by a plurality of current driving devices. The voltage converter 110 receives a power voltage VDD, and generates an operating voltage VCC with a level different to that of the power voltage VDD according to an output of the conversion loop controller 120. A positive input terminal of the amplifier 130 receives a reference voltage Vref, and a negative input terminal thereof receives an output voltage of the voltage selector 140, so that the amplifier 130 accordingly outputs a voltage to control the conversion loop controller 120, wherein the reference voltage Vref is a fixed value. The voltage selector 140 selects and outputs a voltage of a negative terminal of one of LED strings 50_1-50_n. Positive terminals of the LED strings 50_1-50_n receive the operating voltage VCC, and the negative terminals of the LED strings 50_1-50_n are respectively coupled to the current driving unit 150 through switches S1-Sn. The LED strings 50_1-50_n are driven by load currents and the switches are switched according to a dimming signal, so as to implement a dimming operation.
FIG. 1B is a timing diagram of the driving currents of FIG. 1A. Referring to FIG. 1A and FIG. 1B, in the LED driving circuit 100, the PWM technique is generally used to regulate a time t1 for supplying the load currents so as to adjust the brightness of the LED. In other words, in a fixed period T, the longer the time t1 is, the higher the brightness of the LED is. Conversely, the shorter the time t1 is, the lower the brightness of the LED is. However, when the PWM technique is used for dimming, switching operations of the switches S1-Sn lead to a variation of the load currents and the variation of the load currents can lead to a great load variation of the operating voltage VCC, so that the operating voltage VCC output by the voltage converter 110 may have an excessive ripple. Meanwhile, an input current of the voltage converter 100 may also have a great transient variation, which may not only cause a great magnetic field variation of an inductor in the voltage converter 100, but also a regulation capacitor in the voltage converter 100 can be sharply vibrated to generate a shape-changing due to an excessive transient voltage variation, so that the audio noise is generated. Moreover, regarding the driving circuit 100, during the dimming, the switches S1-Sn are simultaneously switched to switch the load currents i1-in, though the current switching operation can cause a severe electromagnetic interference (EMI).